import java.awt.*;
class OffBuffer
{

    private LightContainer container;
    private Image work, back;
    private Dimension offSize;

    public OffBuffer (LightContainer container)
        {
           this.container = container;
        }
    public Image getWork() { return work;}
    public Image getBack() { return back;}

    public Graphics getWorkGraphics() { return work.getGraphics(); }
    public Graphics getBackGraphics() { return back.getGraphics(); }

    public void blitWork() { blitWork(null); }
    public void blitBack() { blitBack(null); }

    public void blitWork( Rectangle clip )
        {
          Graphics screen = container.getGraphics();
          if (clip != null)
            screen.setClip(clip);
          screen.drawImage(work, 0,0, container);
          screen.dispose();


        }

    public void blitBack( Rectangle clip )
        {
           Graphics work = getWorkGraphics();
           if (clip != null)
              work.setClip( clip );
           work.drawImage(back, 0,0, container);
           work.dispose();


        }
    public void update(){
        if (stillValid()) createBuffer();}

    public boolean stillValid()
        {
          Dimension size = container.getSize();
          return (work == null ||
                  size.width != offSize.width ||
                  size.height != offSize.height);

        }
    public void createBuffer()
        {
            offSize = container.getSize();
            work = createOffImage(offSize);
            back = createOffImage(offSize);
        }


    public Image createOffImage(Dimension size)
        {
           Image image = container.createImage(size.width,
                size.height);
           Graphics g = image.getGraphics();
           g.dispose();
           return image;

        }


}